View Full Version : KT88 power valves with low end grip?
Mulling over improving the sound of my valve amp until such time I can afford to replace it with something another level up from what I have.
Cant fault the mid and top end, but after a recent get together at mine and plumbing in a very good solid state amp to my system showed that a bit of extra grip on the lower end will work wonders with my speakers.
Currently using Gold Lion KT88 Reissues which I like a lot, but as I say...if I could get a bit more grip on the bass it owuld be better. :)
Would I get more grip rolling the preamp valves, which are 12AX7 and 12AU7?
Tried a few different ones which made subtle differences, more in the top end than down below.
This made me think swapping out the power valves would change the grunt a bit more...
My speakers are a very efficient 94dB and all cabling is fine.
Any thoughts guys?
Thanks.
Please forgive this, but valves coupled via transformers and passively driven speakers isn't a recipe for TAUT bass. Musical and involving certainly, but not taut at very low frequencies - that's an artefact of the output transformer.
This really shows my ignorance, but can KT120's be substituted? probably not, but I'll chuck it in anyway...
It is a function of the way transformers work, the voltage through the primary winding saturates the magnetic core, which induces voltage into the output winding. Unfortunately this process is not instantaneous, you get something called a hysteresis loop - it is this that makes the low frequencies wooly (the amount of wooliness depending on the density of the core). I replaced the output transformers in my Grant G50 with some comparitively MASSIVE items, and it tightened up the bass loads...
Ali Tait
19-12-2014, 14:47
Aye, as we discussed Gaz, you'd need better output trannies, but by the time you've paid for them you'd be as well going for a new amp...
Ali Tait
19-12-2014, 14:48
If you want to nip over, you can borrow the KEL84 if you like?
Something different to listen to.
not using my valve amp combo either if u want to try it.
Please forgive this, but valves coupled via transformers and passively driven speakers isn't a recipe for TAUT bass. Musical and involving certainly, but not taut at very low frequencies - that's an artefact of the output transformer.
<Cough> Not here it ain’t, muchacho, where bass is as taught and grippy as a very taught and grippy thing! Sometimes you have a habit of talking in absolutes ;)
This really shows my ignorance, but can KT120's be substituted? probably not, but I'll chuck it in anyway...
They can if Gary’s amp can safely take them - and if so, they’ll likely provide much more grip than any KT88s.
Marco.
Please forgive this, but valves coupled via transformers and passively driven speakers isn't a recipe for TAUT bass. Musical and involving certainly, but not taut at very low frequencies - that's an artefact of the output transformer.
This really shows my ignorance, but can KT120's be substituted? probably not, but I'll chuck it in anyway...
How did you work that one out? :scratch: Someone more direct might have responded with "Complete and utter cobblers!" :lol:
Providing that the amp has low output impedance and is well matched to the speakers, plus the bandwidth (ie decent transformers) is good and distortion low, there's NO reason why bass from a valve amp shouldn't be taught and punchy. It's high output impedance resulting in low damping factor combined with low or no negative feedback (which makes the amp less stable into difficult loads) which is a recipe for poor bass control, NOT the fact it's transformer coupled. Substituting one make of KT88 for another WON'T make bass significantly better or worse. The circuit, whether feedback is used, available current, bandwidth and output impedance are far more significant than what brand of valve is used.
Sustituting KT120s into a poorly designed valve amp wont make the bass any better. I use KT77s in mine and have no shortage of deep, taught bass, even into difficult speaker loads.
Valve amps DO NOT usually have low output impedances and this is why (I understand) 1960's and early 1070's speakers were nominally 15 ohms with an absolute minimum of 8 ohms, hence Marco's good perceived results as his Tannoys should be average 11 ohms I think and his amp having very expensive and carefully selected transformers (why else would TD custom amps cost so much - I bet the ironwear accounts for a substantial amount of the cost). VERY few could accommodate humungous old Tannoys in any shape or form though Marco, and I have no idea how the Eddingdales fare in comparison in the bass.
Paul - not sure you're totally right here, since the quality of the transformer seems to almost totally dominates the sonic and technical performance of the valve amp using them. Sure, you can tinker around with the other things, but people who've done this usually ruin the whole thing by trying to shut the stable door after the (transformer) horse has bolted. best get the iron sorted first and then the rest can fall into place. I await your own valve amp with interest :)
Of course, this is a polite thread where the OP was proposing a few things to do. I gave an opinion, which is subject to change and much correction if I'm wrong. I think enough here both now and in the past have commented on how important the output transformer really is and it was talented design that made the 1960's Radfords so advanced for their era, bass distortion and damping factor (what little there is) being ahead of a lot at the time I remember.
Valve amps DO NOT usually have low output impedances and this is why (I understand) 1960's and early 1070's speakers were nominally 15 ohms with an absolute minimum of 8 ohms, hence Marco's good perceived results as his Tannoys should be average 11 ohms I think and his amp having very expensive and carefully selected transformers (why else would TD custom amps cost so much - I bet the ironwear accounts for a substantial amount of the cost). VERY few could accommodate humungous old Tannoys in any shape or form though Marco, and I have no idea how the Eddingdales fare in comparison in the bass.
Indeed, Dave, but the point is that not all valve amps have wooly bass and poor grip (in comparison with SS amps), which is the way your original comment came across and the picture you were painting, as if it was a ‘universal fact’.
Yes, with valve amps ‘it’s all in the iron’, but if you afford to buy a valve amp with suitably good/large transformers, and most importantly, partner it with the right speakers, then wooly bass is a non-issue. Again, it doesn’t matter that not many can accommodate humungous old Tannoys (or whatever else is chosen to get the job done) - the point is that the technology itself is capable of it!
:exactly:
In 30-odd years of using hi-fi equipment, and mostly solid-state amps (some huge behemoths amongst them), I’ve never had such deep, tight, tuneful and extended bass in my system, as I have now - and that’s with a valve amp in the equation.
Marco.
Solid state designers may disagree with you (no, not THAT one :)!!!!!) and your speakers present a special case anyway, but all the reviews I've read have indicated much the same thing - when frequencies plummet - distortion usually rises (not the nasty type bad ss exhibits I grant you) and max power suffers a bit. Whether this is audible or in practise an issue at all is a moot point and I'm the last one to push it.
PLEASE remember Marco, you have special speakers, even by Tannoy standards and the amp you use was designed and built by a specialist who seems to have an instinctive 'feel' for the task at hand. Sorry if I was appearing to slag off ALL valve amps made this way - wasn't what I was trying to say at all. I mean, I've been very pleasantly surprised by some of the better Icon Audio valve amps - however they've been optimised, the bass on these is very taut and tuneful :)
Hi Dave,
Solid state designers may disagree with you (no, not THAT one :)!!!!!) and your speakers present a special case anyway, but all the reviews I've read have indicated much the same thing - when frequencies plummet - distortion usually rises (not the nasty type bad ss exhibits I grant you) and max power suffers a bit. Whether this is audible or in practise an issue at all is a moot point and I'm the last one to push it.
Precisely - and that last bit nails it! I couldn’t give a monkey’s toss what the measurements say; for an audio enthusiast, it’s what happens in the real world (when you listen) that counts! :)
PLEASE remember Marco, you have special speakers, even by Tannoy standards and the amp you use was designed and built by a specialist who seems to have an instinctive 'feel' for the task at hand. Sorry if I was appearing to slag off ALL valve amps made this way - wasn't what I was trying to say at all.
No worries, mate. It’s just that’s how it was coming across a bit.
Understand that there are two things I’m extremely passionate about in audio, and will fight to my last breathe to defend, and that’s a) the misguided notion, held by many so-called ‘audiophiles', that the Technics SL-1200/1210 is ‘just a DJ deck’, not worthy of modifying - and breaking down the whole ‘belt-drive brainwash’, especially here in the UK, and b) that valve amps automatically sound ‘warm and wooly’!!
Therefore, when I sense even an inking of that blinkered bullshite, I’ll always be there to rip it to shreds, for the fallacious pish that it is. You can call it my ‘pet topics’… ;)
Marco.
Valve amps DO NOT usually have low output impedances and this is why (I understand) 1960's and early 1070's speakers were nominally 15 ohms with an absolute minimum of 8 ohms, hence Marco's good perceived results as his Tannoys should be average 11 ohms I think and his amp having very expensive and carefully selected transformers (why else would TD custom amps cost so much - I bet the ironwear accounts for a substantial amount of the cost). VERY few could accommodate humungous old Tannoys in any shape or form though Marco, and I have no idea how the Eddingdales fare in comparison in the bass.
Paul - not sure you're totally right here, since the quality of the transformer seems to almost totally dominates the sonic and technical performance of the valve amp using them. Sure, you can tinker around with the other things, but people who've done this usually ruin the whole thing by trying to shut the stable door after the (transformer) horse has bolted. best get the iron sorted first and then the rest can fall into place. I await your own valve amp with interest :)
Of course, this is a polite thread where the OP was proposing a few things to do. I gave an opinion, which is subject to change and much correction if I'm wrong. I think enough here both now and in the past have commented on how important the output transformer really is and it was talented design that made the 1960's Radfords so advanced for their era, bass distortion and damping factor (what little there is) being ahead of a lot at the time I remember.
Sorry Dave but I just don't know where get your facts from. To suggest that a majority of well designed valve amps are high output impedance is just plain nonsense. Poorly designed ones maybe (read "most amps from Chinese sources and most single ended amps perhaps). A vast majority of well designed PP valve amps and even SE amps using some negative feedback will have very commendable control of bass. There's no issue whatsoever with a decent valve amp with appropriate tappings driving 4 Ohm loads for example. I'd be interested in how many well designed valve amps you've measured or actually understand the specs for? They don't have to be exemplary designs by some mystic genius sat in a mountain cave for goodness sake. The vast majority of valve amps (PP) are designed around one of just 4 or 5 basic circuits, the Mullard 520 being a classic example. Properly implemented (like Radfords for example) the output impedance can be way less than half an ohm. Sometimes much less (I think Radfords are around 0.2 Ohms). I also don't understand how you've managed to misinterpret what I've said about transformers since I was actually agreeing with what you've written above! ie transformer design and quality is of paramount importance!
My own amp is the Emille Ki40L. It is properly designed, it does bass with exceptional control and extension and doesn't conform to the misguided misconceptions that some have about all valve amps being "woolly" in the bass. Sure, most SS amps have low output impedance and low distortion at higher outputs, but many also have HIGHER distortion at lower outputs and when they clip, they sound horrible (and risk damage to tweeters), so you really do need to up-spec their output compared with a valve amp (which mostly distort in a euphonic way when pushed hard).
There are good and bad examples of both SS and valve amps but it's plain wrong to generalise about bass control with valve amps as a design genre.
All I'm pointing out is that it's incorrect to make such generalisations Dave without understanding some fundamental issues of design and execution.
walpurgis
19-12-2014, 22:39
I have examples of valve amplification which exhibit opposite bass characters. One with the bass being full of 'bloom' and rather slack (but a lovely sounding amp nonetheless) and another which has tight bass grip with fine extension, at least as good as any solid state I've used. Driving large Tannoys I might add. So obviously valve amps vary.
Oh, and I have heard solid state amps with slack and ill defined bass. Owned some in fact. Big ones too.
Dunno, all I know is mine sounds great with plenty tight bass. oh and it is Chinese;)
Oh, and I have heard solid state amps with slack and ill defined bass. Owned some in fact. Big ones too.
And me! As ever, in amplifier design, implementation and attention to detail is far more important than the choice of topology. Most importantly, there are no absolutes! :)
Marco.
Get some 300B's and be happy! :lol:
In push-pull mode, aye! ;)
Marco.
Ninanina
19-12-2014, 23:10
and b) that valve amps automatically sound ‘warm and wooly’!!
Marco.
I've had a few valve amps now and while some have certainly sounded a bit "warm and wooly" my current Oto does not have these traits at all, well not to my ears anyway ;)
Radford Revival
19-12-2014, 23:10
If you take 'grip' to mean 'output impedance', then that is down to the design of the amplifier, and in the case of a valve amplifier, not necessarily down to the type of output valves used. If a design is good and uses decent output transformers that lend themselves to the use of enough feedback, then a "mere" EL84 amplifier within its power limit can have just as much 'grip' as a KT88 or KT120 amplifier. It's tempting to see big valves with big anodes and equate that to bass control, but it's the output impedance that matters.
Low output impedance is achieved via feedback. This is either via deliberate feedback loops or fortuitously from the internal feedback present in low impedance valves such as triodes (though using triodes alone is often insufficient).
walpurgis
19-12-2014, 23:17
Get some 300B's and be happy! :lol:
Er, they only do midrange don't they? :D
Just for the record, by using feedback you can make valve amps with any output impedance you want, positive of negative. In the 50's there was a trend for making amps with adjustable output impedance to match to the speakers. S
Solid state has got people into the habit of talking about output impedance as if it was purely resistive, its not, there is a frequency component as well, and any real speaker will present a varying phase load onto the amp. What (as has been said) is important for the bass response is the power bandwidth of the amp and its ability to handle complex loads. Both of these are dependent (at the LF end) on the available flux the output transformer can support. As frequency drops, the flux increases quickly so the flux needed for 1W at 100Hz is 4 times (I think, not my area of expertise, but its all in RDH) less than 50Hz. and for a given core material the max flux is governed by the size of the core. Of course as the core gets larger, so does the leakage inductance so the HF response suffers, so to get both you need a large core AND a well designed (generally complex) winding scheme. Neither of those come cheap.
Swapping kt88 for kt120, will because of the slightly lower plate resistance of the kt120's present a lower resistive output impedance, but the core of the transformer wont magically be able to handle more flux.
Getting the output stage balanced will help, so at idle the net flux is zero, any imbalance in a push pull output transformer will quickly use up the available flux leaving none for the music, or at lease leaving none in one direction increasing the distortion levels.
Er, they only do midrange don't they? :D
No they are happy down to DC. In fact 300b's can make a good valve for a shunt regulated power supply.
But what you mean is its not simple to make a wide bandwidth single ended 300b amp. But it is possible. You need a good power supply and good output transformers for the LF, and driving the 300b with a high enough current to avoid slew limiting, AND good output transformers for the HF response. But yes the midrange will take care of itself. :-)
+1 In addition to what Nick has just said, this is a good link to read about how o/p impedance affects amplifier response: http://www.transcendentsound.com/Transcendent/Amplifier_Output_Impedance.html
Hi Nick,
Of course as the core gets larger, so does the leakage inductance so the HF response suffers, so to get both you need a large core AND a well designed (generally complex) winding scheme. Neither of those come cheap.
Would that explain the potential advantages of having a valve amp with hand-wound transformers? :)
Marco.
Sorry Dave but I just don't know where get your facts from.
There are good and bad examples of both SS and valve amps but it's plain wrong to generalise about bass control with valve amps as a design genre.
All I'm pointing out is that it's incorrect to make such generalisations Dave without understanding some fundamental issues of design and execution.
I come from an era of damping factor levels of 1000+ in the bass, so any transformer coupled amp is going to be at an 'academic' if not a 'sonic' disadvantage. Yes, it's a complicated subject and no, I haven't got the maths skills to discuss electronic design, let alone design anything 'special' for myself or anyone else. Back in the mid 70's when specs ruled, you could hear what 'damping factor' could 'supposedly do' for some loudspeakers, although I think it's Doug Self who's more recently rubbished the concept and this has been leapt on by Harbeth and others to help justify their complex crossovers which supposedly 'decouple' the amp from the speaker more and more as the frequencies rise.
Gazjam, sincere apologies if you feel I've crapped all over your thread with, well, more and more crap! I hope you find a cost effective solution. Even moving the speakers about may possibly help?
Back in the mid 70's when specs ruled, you could hear what 'damping factor' could 'supposedly do' for some loudspeakers...
You could also hear how ‘grey’ and musically bland they were too, as there’s nothing like applying shed loads of feedback (no matter how much bass control it provides), for killing the music! ;)
Marco.
P.S Thanks goodness these days specs no longer rule.
tubehunter
20-12-2014, 10:26
You could also hear how ‘grey’ and musically bland they were too, as there’s nothing like applying shed loads of feedback (no matter how much bass control it provides), for killing the music! ;)
Marco.
P.S Thanks goodness these days specs no longer rule.
But your Croft pre does just that!
http://i514.photobucket.com/albums/t350/halx00/IMG_6058_zps66dafc95.jpg (http://s514.photobucket.com/user/halx00/media/IMG_6058_zps66dafc95.jpg.html)
It’s not solid-state, though ;)
Maybe it’s just ‘coincidence’, but I’ve heard loads of SS amps, which employ high levels of feedback, and to me they all sounded very ‘flat’ and musically bland, something that is the complete antithesis of the Croft!
Marco.
Firebottle
20-12-2014, 10:51
...ah but that's not shed loads of feedback as the open loop gain isn't huge anyway. Plus it's not 'overall' negative feedback.
:) Alan
I come from an era of damping factor levels of 1000+ in the bass, so any transformer coupled amp is going to be at an 'academic' if not a 'sonic' disadvantage. Yes, it's a complicated subject and no, I haven't got the maths skills to discuss electronic design, let alone design anything 'special' for myself or anyone else. Back in the mid 70's when specs ruled, you could hear what 'damping factor' could 'supposedly do' for some loudspeakers, although I think it's Doug Self who's more recently rubbished the concept and this has been leapt on by Harbeth and others to help justify their complex crossovers which supposedly 'decouple' the amp from the speaker more and more as the frequencies rise.
Gazjam, sincere apologies if you feel I've crapped all over your thread with, well, more and more crap! I hope you find a cost effective solution. Even moving the speakers about may possibly help?
Not at all Dave, it's led to some interesting discussion.
All good info guys, thanks.
Ali had mentioned it's all down to the trannies but unfortunately without throwing a sizeable chunk of wedge at it I'm looking to identify if there's anything I can do meantime. :)
When my amp is up and running the power and output transformer housings don't get very hot, only slightly warm...
Might this indicate that there's sufficient headroom there to (say) fit the KT120's, given differences in heater current etc?
Ali had a look at my amp and did some measuring with a bit of input from Nick, and it seemed to measure pretty ok? It's a Chinese amp but measured with proper UK voltage etc.
Question?
Appreciate that without putting it on a measuring bench You can't be exactly sure, but would the trannies not getting very warm with the KT88's in place and the amp having "measured well" be an indication of the feasibility of swapping in some KT120's in?
I totally take on board Nick and Paul what you said about it being down to impedance, circuit design and the transformers, but wonder there may be advantages with the different valves, even if not earth changing. :)
Thanks.
You could also hear how ‘grey’ and musically bland they were too, as there’s nothing like applying shed loads of feedback (no matter how much bass control it provides), for killing the music! ;)
Marco.
.
That's just as much a generalisation as saying all valve amps sound wooly in the bass. I know what you mean about grey sounding SS amps but IME much of the time this is for other reasons (probably the pre-amp section in an integrated).
One of the main reasons I don't use a valve power amp is because good ones are bloody expensive because as has been said previously you need really good traffos and they are not cheap. I don't think I am sacrificing much using an SS power amp with negative feedback at a tenth of the cost.
I've heard the Edingdale speakers like Gaz is using driven by a Chinese valve amp and it let them down badly I thought.
Hi Nick,
Would that explain the potential advantages of having a valve amp with hand-wound transformers? :)
Marco.
Well, not exactly, you could hand wind a crap transformer if you wanted. But the chances if the builder is going to go to that trouble and expense then the odds are that the design will be better in the first place. Just compare the winding spec of the original Willamson or a Radford O/P transformer and something cheap and simple and you will see what I mean. Lots of layers take lots longer to wind and connecting them all together is costly (and not something that you would try and automate).
...ah but that's not shed loads of feedback as the open loop gain isn't huge anyway. Plus it's not 'overall' negative feedback.
:) Alan
Well, with respect, how much more "overall" could you get. From the output to the input. Its just a anode follower done over two stages, by definition it works by the action of feedback.
That's just as much a generalisation as saying all valve amps sound wooly in the bass. I know what you mean about grey sounding SS amps but IME much of the time this is for other reasons (probably the pre-amp section in an integrated).
Yeah, sure, but I wasn’t saying that all SS amps sound that way, simply the ones I’ve heard that employ shed-loads of feedback. You know full well that I rated, rather highly, a certain SS amp I heard at Jason’s ;)
And there are others in that category, too!
Marco.
Radford Revival
20-12-2014, 11:30
Not at all Dave, it's led to some interesting discussion.
All good info guys, thanks.
Ali had mentioned it's all down to the trannies but unfortunately without throwing a sizeable chunk of wedge at it I'm looking to identify if there's anything I can do meantime. :)
When my amp is up and running the power and output transformer housings don't get very hot, only slightly warm...
Might this indicate that there's sufficient headroom there to (say) fit the KT120's, given differences in heater current etc?
Ali had a look at my amp and did some measuring with a bit of input from Nick, and it seemed to measure pretty ok? It's a Chinese amp but measured with proper UK voltage etc.
Question?
Appreciate that without putting it on a measuring bench You can't be exactly sure, but would the trannies not getting very warm with the KT88's in place and the amp having "measured well" be an indication of the feasibility of swapping in some KT120's in?
I totally take on board Nick and Paul what you said about it being down to impedance, circuit design and the transformers, but wonder there may be advantages with the different valves, even if not earth changing. :)
Thanks.
Changing the output valves to KT120 will have little to no effect on the bass performance of the amplifier, and potentially stress the heater supply. The warmth of the output transformers is down to the DC bias current (a relatively small heating effect) and absorbed heat from the rest of the amp and has no relation to "headroom". The fact that the mains transformer isn't hot doesn't necessarily give any indication as to the current capacity of the heater windings.
Well, not exactly, you could hand wind a crap transformer if you wanted. But the chances if the builder is going to go to that trouble and expense then the odds are that the design will be better in the first place. Just compare the winding spec of the original Willamson or a Radford O/P transformer and something cheap and simple and you will see what I mean. Lots of layers take lots longer to wind and connecting them all together is costly (and not something that you would try and automate).
Yup, sure. I wasn’t saying that hand-wound transformers were a ‘magic bullet’, but rather, done properly using quality transformers in the first place, by someone who knows what they’re doing, they might have the edge on machine-wound ones of the same quality.
The chap who hand-wound the transformers on my Copper amp used to work for G.E.C, so I’m presuming he knew what he was doing ;)
I can certainly confirm that the quality of the transformers themselves are top-notch.
Marco.
Thanks Will,
Good info.
I know there's no magic bullet with this, just wondering what my options are.
Not a major issue tbh, amp sounds great and I really enjoy the sound I am getting...but know that the Edingdales will really benefit from a better amp.
Come the lottery win (!) I'll ask Mr G if he'd be kind enough to build me something to drive the eddingdales better.
In the meantime...any money spent on tweaking my current amp should be put towards a new amp.
Thanks for all the replies, job done. :thumbsup:
Marco - I think you are jumping to conclusions blaming the negative feedback. Your subjective opinion based on listening is the amps sound grey. The technical explanation is irrelevant really so I don't see the point in speculating. Even experienced audio engineers have a difference of opinions on the deleterious effects of negative feedback so civilians like us have no chance of making definitive statements about it.
Hi Will,
Changing the output valves to KT120 will have little to no effect on the bass performance of the amplifier, and potentially stress the heater supply.
I hear you, but perhaps that’s the case on amps not designed to take full advantage of the benefits of KT120s? I believe that some KT88 designed valve amps have to be modified, in terms of the heater supply, in order to safely accommodate them.
When I moved from using GEC KT88s (and Winged-C Svetlanas before that), in my Copper amp, to KT120s, the bass response was considerably improved, both in control and extension, not to mention the sonic benefits in other areas, which is why the use of KT120s now on my amp is a no-brainer.
Therefore, if that applies to my Copper amp, surely it could also do in others, once (if necessary) they’ve been modified to take KT120s properly?
Marco.
I come from an era of damping factor levels of 1000+ in the bass, so any transformer coupled amp is going to be at an 'academic' if not a 'sonic' disadvantage. Yes, it's a complicated subject and no, I haven't got the maths skills to discuss electronic design, let alone design anything 'special' for myself or anyone else. Back in the mid 70's when specs ruled, you could hear what 'damping factor' could 'supposedly do' for some loudspeakers, although I think it's Doug Self who's more recently rubbished the concept and this has been leapt on by Harbeth and others to help justify their complex crossovers which supposedly 'decouple' the amp from the speaker more and more as the frequencies rise.
Gazjam, sincere apologies if you feel I've crapped all over your thread with, well, more and more crap! I hope you find a cost effective solution. Even moving the speakers about may possibly help?
Actually Dave, having a damping factor of 1000 is totally academic and quite meaningless in itself. You are forgetting that the impedance of the speaker cable and crossover is additive to the output impedance of the amplifier when calculating damping factor, so for all intents and purposes, all you're looking for in most cases as a DF of perhaps 15 to 20 for perfectly adequate control of most efficient speaker designs.
Lets use an example. If a speaker uses a 3rd or 4th order crossover with a total additive impedance (mainly from the inductors in series with the circuit) of about 1 Ohm (which may not be unrealistic with some designs) and the speaker cable adds say 0.1 ohms loop resistance, then its easy to see how your amp with a DF of 1000 suddenly appears as one with a much lower damping factor. If the output impedance were say 0.05 Ohms to start with, then its new DF into a typical 6 Ohm load is now reduced from a rated 120 to just 5.2! OK...I admit to having over simplified this example, but it does still serve to highlight that very high damping factors in themselves are not always the be all and end all if the connected loads are very difficult and inefficient.
One might point out that speakers have rising impedance with frequency, but bass usually lies in the lower end of the impedance curve, just above system resonance for most loudspeaker designs.
This is one reason for using low DCR inductors in the signal path. The total load from my 3rd order Rhapsody speaker crossovers for example is still under 0.5 Ohms DC. There are other considerations too such as available current (power supplies and amp rating are a consideration) so all we're trying to point out (read Nick and Wills' posts above) are that things are not as clear cut and simple as they may first appear, so generalisms about valve amps and in particular statements like "valve amps can't do bass like SS amps can" are a a little misguided. It's all down to design and implementation. As Nick points out, a lot of the issue with cheaper valve amps is the quality of the output transformers. Poor ones may not have the bandwidth and may be much higher in distortion. One statement that I think it's safe to say is that a good valve amp costs more to make than an equivalent SS amp of similar specification, and had you said that, I think there'd have been general agreement.
Back to the OP. Swapping out different valves may change the sonic flavour of your amp but unfortunately, as the rules of physics cannot be altered wrt to the feeback used or output transformers, you may not be able to do very much with bass control. It may be possible to have someone like Nick or Will have a look at the feedback employed and perhaps tweak the amp, but any changes to upspec output transformers may be quite expensive. If you have variable tappings, you could try the 4 Ohm outputs in place of the 8 Ohm ones and see if this brings any improvements?
Marco - I think you are jumping to conclusions blaming the negative feedback. Your subjective opinion based on listening is the amps sound grey. The technical explanation is irrelevant really so I don't see the point in speculating. Even experienced audio engineers have a difference of opinions on the deleterious effects of negative feedback so civilians like us have no chance of making definitive statements about it.
Well, I can only report on what I hear, and every time an SS amp I’ve been listening to has employed a heavy use of feedback, to my ears, it has sounded ‘grey’ and sterile, and exhibited a tendency to ’strangle’ the music. Of course, that’s only my subjective opinion, but it’s a valid one nonetheless, as it’s based on considerable experience of listening to SS amps of all types.
No ‘definitive statements’ here, though :)
Marco.
Back to the OP. Swapping out different valves may change the sonic flavour of your amp but unfortunately, as the rules of physics cannot be altered wrt to the feeback used or output transformers, you may not be able to do very much with bass control. It may be possible to have someone like Nick or Will have a look at the feedback employed and perhaps tweak the amp, but any changes to upspec output transformers may be quite expensive. If you have variable tappings, you could try the 4 Ohm outputs in place of the 8 Ohm ones and see if this brings any improvements?
Thanks Paul, I do have 4ohm outputs on my amp, I'll try those, might prefer the flavour.
Cheers.
Radford Revival
20-12-2014, 12:28
Hi Will,
I hear you, but perhaps that’s the case on amps not designed to take full advantage of the benefits of KT120s? I believe that some KT88 designed valve amps have to be modified, in terms of the heater supply, in order to safely accommodate KT120s.
When I moved from using GEC KT88s, in my Copper amp, to KT120s, the bass response was considerably improved, both in control and extension, not to mention the sonic benefits in other areas, which is why the use of KT120s now on my amp is a no-brainer.
Therefore, if that applies to my Copper amp, surely it could also do in others, once (if necessary) they’ve been modified to take KT120s?
Marco.
Hi Marco,
I'm not familiar with the topology used in the Copper amps but the difference could have been down to your particular set of KT120s having a slightly higher Gm (transconductance / gain) value than your existing KT88s, which may have lent itself to making enough difference to the output impedance to make a tangible difference to the sound. This is just broad speculation on my part.
Given some KT88s that had similar gain at the same operating point I would imagine the differences would have appeared with those too. I would personally be very very hesitant to say the differences came about "because" KT120s were used, from a purely technical standpoint.
With a high feedback amplifier the differences would be largely swamped by the feedback. Amps with lower or no feedback are more influenced by the condition / gain of the valves.
This is all speaking from a purely technical standpoint of course.
Hi Marco,
I'm not familiar with the topology used in the Copper amps but the difference could have been down to your particular set of KT120s having a slightly higher Gm (transconductance / gain) value than your existing KT88s, which may have lent itself to making enough difference to the output impedance to make a tangible difference to the sound. This is just broad speculation on my part.
Given some KT88s that had similar gain at the same operating point I would imagine the differences would have appeared with those too. I would personally be very very hesitant to say the differences came about "because" KT120s were used, from a purely technical standpoint.
With a high feedback amplifier the differences would be largely swamped by the feedback. Amps with lower or no feedback are more influenced by the condition / gain of the valves.
This is all speaking from a purely technical standpoint of course.
Hi Will,
Thanks for sharing that, and noted. Thing is, I’ve noticed the same thing when testing other KT88s against different KT120s, and the results in my amp were the same. Also, the only variable when I first did it, was the KT120s themselves - nothing else was changed in the amp or my system to cause the difference in sound; simply one set of valves swapped out for another.
I’ll ask AnthonyTD to join the discussion, as he made my amp and may be able to throw some more light on why I heard what I did. Incidentally, I believe the same thing happened in his Soul Amp, when he swapped from KT88s to KT120s. It certainly also happened in two other Copper amps I’ve heard belonging to Steve Toy and Ian Walker, in that moving from KT88s to KT120s was a no-brainer, sonically.
Marco.
Firebottle
20-12-2014, 16:48
Well, with respect, how much more "overall" could you get. From the output to the input. Its just a anode follower done over two stages, by definition it works by the action of feedback.
Point taken Nick with just that circuit feedback. I should have clarified as to meaning the whole of the amplifier chain up to the speaker outlets.
:rolleyes: Alan
anthonyTD
20-12-2014, 16:57
Hi all,
If you compare for example the original svetlana winged C MA/V and plate resistance, to the KT120 you may come to the same conclusions as me, also, look up the parameters of the KT150, and again you should also realise why the KT150 seems to have the opposite affect when placed in the same circuit! When it comes to grip and sheer effortless' musical presentation' the KT120 IMHO wins hands down, even in a circuit [that isnt quite optimised for its parameters].
However, When it comes down to over-all sonic presentation' and musicality' the original GEC KT88 is still [again IMHO] a hard act to follow.:)
A...
Interesting, Anthony… Could you explain from a technical point of view why the bass performance (control and extension) significantly improved on my Copper amp, by moving from KT88s to KT120s (along with that of the other Copper amps mentioned)?
Didn’t the same thing not also happen with your Soul amps? If so, one would presume that there must be something about the design of the KT120 (when properly exploited/executed in the right amp) that provides superior sonic performance to that of a KT88, all else being equal... :)
Marco.
anthonyTD
20-12-2014, 17:06
There are two important diffrences in the three valves in question, MA/V or GM, and internal plate resistance!
Interesting, Anthony… Could you explain from a technical point of view why the bass performance (control and extension) significantly improved on my Copper amp, by moving from KT88s to KT120s? :)
Marco.
Lol - you’ve lost me there! I’ll leave that one for the technical boys to fathom. All I know is that, to my ears with my amp, KT120s piss all over any KT88s I’ve used, although the GECs, as you say, have a certain ‘magic’ of their own.
Marco.
spendorman
20-12-2014, 18:12
The chap who hand-wound the transformers on my Copper amp used to work for G.E.C, so I’m presuming he knew what he was doing ;)
That made me laugh, so did I, and it was as an Engineer.
So you were an engineer for GEC, Alex? :)
Marco.
spendorman
20-12-2014, 18:28
So you were an engineer for GEC, Alex? :)
Marco.
Yes, GEC Marconi, later became BAE Systems, I was only there for 20 years.
Back to the topic, all my valve amps are vintage British ones, generally they do have slightly less controlled bass, but very nice sounds, the exception is the Radford STA25 III, bass is pretty controlled from this. Reasons already given in previous posts.
Thanks for understanding and extra info folks and hope all the technical stuff hasn't swamped the intentions of the OP... The most any of us non-tech bods can do is to try different things (within reason) and see if the music is perceived any differently on a long-term basis (I still believe our ears can change daily, so without a known reference, a longer term may well be needed to fully confirm initial impressions).
My Crofty-Quad II's are way outclassed these days, yet they have a lovely beguiling 'gloriously coloured' nature to them that musically wins every time. I'm so worried about wearing out the original GEC KT66's I don't use them very often.
I think I have the original review on Marco's amp, or at least it's sibling (I think only two or three 'Copper' amps were ever made???).
Actually Gaz, it may one day be a good idea to let Anthony take a look at your amp. Engineers often like a gander at 'the competition' ;)
spendorman
20-12-2014, 19:10
Thanks for understanding and extra info folks and hope all the technical stuff hasn't swamped the intentions of the OP... The most any of us non-tech bods can do is to try different things (within reason) and see if the music is perceived any differently on a long-term basis (I still believe our ears can change daily, so without a known reference, a longer term may well be needed to fully confirm initial impressions).
My Crofty-Quad II's are way outclassed these days, yet they have a lovely beguiling 'gloriously coloured' nature to them that musically wins every time. I'm so worried about wearing out the original GEC KT66's I don't use them very often.
I think I have the original review on Marco's amp, or at least it's sibling (I think only two or three 'Copper' amps were ever made???).
Actually Gaz, it may one day be a good idea to let Anthony take a look at your amp. Engineers often like a gander at 'the competition' ;)
I have Quad II's as well, yes lovely sound, and not outclassed if used with suitable speakers, perhaps some ESL57's or old Tannoy Dual Concentrics. Those GEC KT66's should last a long time. I've had mine since the early 70's. Mind you, I do have a few new spares.
There are two important diffrences in the three valves in question, MA/V or GM, and internal plate resistance!
So, Nick or Will (or anyone else), did you pick up on what Anthony is getting at here or not? The silence is deafening! ;)
Marco.
The Black Adder
21-12-2014, 08:25
Yes, GEC Marconi, later became BAE Systems, I was only there for 20 years.
Back to the topic, all my valve amps are vintage British ones, generally they do have slightly less controlled bass, but very nice sounds, the exception is the Radford STA25 III, bass is pretty controlled from this. Reasons already given in previous posts.
It would be great to hear your experiences working at GEC... :) If you could start a new thread with that it would be most interesting I'm sure.
It would be great to hear your experiences working at GEC... :) If you could start a new thread with that it would be most interesting I'm sure.
I am sure they would be interesting, I worked with BAE Systems (Samlesbury) installing and commissioning a £2e6 hydraulic press 25 years ago, I I have many stories from that, but not so many that would be HiFi related.
My rather clumsy point is GEC did a lot more than make kt88's
So, Nick or Will (or anyone else), did you pick up on what Anthony is getting at here or not? The silence is deafening! ;)
Marco.
Oh, sorry, I didnt know it needed more saying, the transconductance of the valves will make a difference, assuming fixed gain, increased transconductance will mean lower plate resistance, and as I mentioned lower ra will translate to lower reflected impedance out of the amp. But depending on the amount of feedback in use that will have more or less effect on what the speaker sees.
kt88, gm 11ma/V
kt120, gm 12.5 ma/V
kt150, gm12.6 ma/V
Note the above is just from spec sheets, not measured them myself.
My mums cousin who was just like a brother as they lived in adjacent houses worked for Gec Marconi as a draftsman in Fife for years before he got Parkinsons. He was a fabulous draftsman and person and loved the company. they held his job open for him for years in the hope he would come back. think it was to do with defence systems
spendorman
21-12-2014, 10:50
It would be great to hear your experiences working at GEC... :) If you could start a new thread with that it would be most interesting I'm sure.
I was concerned with Electronics mainly, there were some related subjects, Optics, Thermodynamics, Hydraulics, Chemistry etc.
As Lurcher said; "GEC did a lot more than make kt88's".
And discussing anything in detail might not be appropriate, as information was / is covered by the Official Secrets Act.
What was just as interesting as the very advanced Engineering / Electronics, was the high level of internal politics.
Googleing "Marconi mystery", might give rise to another thread.
Oh, sorry, I didnt know it needed more saying, the transconductance of the valves will make a difference, assuming fixed gain, increased transconductance will mean lower plate resistance, and as I mentioned lower ra will translate to lower reflected impedance out of the amp. But depending on the amount of feedback in use that will have more or less effect on what the speaker sees.
kt88, gm 11ma/V
kt120, gm 12.5 ma/V
kt150, gm12.6 ma/V
Note the above is just from spec sheets, not measured them myself.
Cheers. So, meandering past the technicalities, and this is aimed at both Anthony and you, is that the likely reason why the bass response with my Copper amp was significantly improved when I moved from using KT88s to KT120s?
I’m trying, in layman’s terms, to obtain an understandable technical reason that explains what I heard.
This will help others who are contemplating using KT120s (with a KT88 designed valve amp) to decide if the potential benefits are worthwhile, and also provide certain doubters out there, who seem adamant that KT120s are no better than KT88s, with valid proof that real, and very worthwhile, sonic improvements are possible by making the switch, as I certainly wasn’t ‘imagining’ it ;)
Cheers :)
Marco.
anthonyTD
21-12-2014, 11:32
Marco, Nick undoubtably understands why KT120's make a diffrence as his explanation bears out, the output impedance of the amp is affected due to the GM, OR MA/V, diffrence, combined by the lower internal plate resistance of KT120's compared to KT88, This is the main reason you were able to lower the over-all feedback of the amplifier, gaining more presence, without sacrificing bass grip and control. The interesting one is the KT150, even though the Transconductance or MA/V is similar, or identical to KT120's the internal plate resistance/impedance is actualy higher than a KT88...
A...
Thanks, Anthony, I was just trying to link the technical points Nick and you made, in reference to KT120s, with the subjective sonic improvements I heard, which you’ve now done. That should hopefully now satisfy the ‘doubting Thomas’’ out there and/or 'pro-KT88 fanboys’ that very real sonic improvements can be gained with the use of KT120s ;)
I know you don’t rate the KT150, and the observation you’ve made is interesting, but at some point (as we discussed) I’ll like to try a matched quad for myself, especially as it’s a straightforward swap in the Copper amp :)
Marco.
anthonyTD
21-12-2014, 12:14
No worries.:)
A...
Cool. Perhaps it’ll give Will some food for thought regarding this comment:
Changing the output valves to KT120 will have little to no effect on the bass performance of the amplifier.
Or perhaps it just requires some qualifying? Anyway, we’re all on a continual learning curve in this game :)
Marco.
Radford Revival
21-12-2014, 13:13
Cool. Perhaps it’ll give Will some food for thought regarding this comment:
Or perhaps it requires some qualifying? Anyway, we’re all on a continual learning curve in this game :)
Marco.
Gm varies quite a bit with operating point so a 10% difference doesn't mean much in terms of a static value given in the datasheet - you'd have to interpolate from the curves or directly measure the actual Gm at the actual operating point as used in a particular amplifier. In the KT120 datasheet I'm looking at the 12.5mA/V Gm value comes about when the valve is running with 225V on the screen, 400V on the plate with a specified anode current range of 135-165mA, which means approximately 60W of plate dissipation. A KT88 simply cannot run at that operating point However, for the sake of argument, I just subjected a real KT88 on our valve tester to a very similar operating point (for a couple of seconds) and as it happens measured a Gm of around about 12.5mA/V. It depends where you are on the curves.
It also depends on the topology of the amplifier, but if used in a feedback amplifier then differences in gain from a different valve (ie just newer or simply a different example) in the driver stage could then make back the difference in open loop gain cause by a differing output valve.
Lol - I understood about 55% of that - maybe 60% if I’m being generous! :eyebrows:
Anyway, for me it just shows how careful we all have to be of making ‘absolutist statements’ in audio, when so many unconsidered variables can sometimes come into play :)
Anthony, for example, is not prone to making these types of observations for nothing:
When it comes to grip and sheer effortless' musical presentation' the KT120 IMHO wins hands down, even in a circuit [that isnt quite optimised for its parameters].
The bottom line, however, in terms of this thread (and Gary’s original question) is that KT120s *could* provide a better solution for him (and others) than any KT88s, hence why I recommended them - and indeed continue to recommend them to people who use KT-88 designed valve amplifiers, looking for a potential upgrade, topology of the amplifier depending.
Marco.
Lol - I understood about 55% of that - maybe 60% if I’m being generous! :eyebrows:
Anyway, for me it just shows how careful we all have to be of making ‘absolutist statements’ in audio, when so many unconsidered variables can sometimes come into play :)
Anthony, for example, is not prone to making these types of observations for nothing:
The bottom line, however, in terms of this thread (and Gary’s original question) is that KT120s *could* provide a better solution for him (and others) than any KT88s, hence why I recommended them - and indeed continue to recommend them to people who use KT-88 designed valve amplifiers, looking for a potential upgrade, topology of the amplifier depending.
Marco.
BUT....the KT120 draws between 100 and 300 mA more filament current than a KT88 so one should never directly substitute a KT120 for a kT88 before first checking with the manufacturer. Again, one must be mindful of generalising about these things when making recommendations. Valve amps do not conform to any sort of universal specification any more than SS amps do, so it is a dangerous thing to ever make assumptions about what is or isn't safe in terms of valve rolling. The danger here is that by doing so, an amp which isn't designed to take either valve will likely burn out part of the power supply windings due to overcurrent by dropping in a KT120 for a KT88. (There's also the question of circuit design and gain and distortion aside, some circuits may not gain the perceived sonic benefits even if the valves can safely be substituted but that's a subjective assessment for such amp owners to make for themselves as they may well prefer one to the other. I know of some who swear by NOS GEC KT88s and wouldn't use anything other than these). I do note your use of the qualification "...could..." though Marco
Indeed, Paul (I agree with all of that), which is why I wrote way back in post #7, when the question was asked: "This really shows my ignorance, but can KT120's be substituted? probably not, but I'll chuck it in anyway…”
They can if Gary’s amp can safely take them - and if so, they’ll likely provide much more grip than any KT88s.
…and so I stand by all of the above statement, the first part and the last part (certainly that’s the case with my own amp) :)
Marco.
Not going with KT120's.
Indeed they MIGHT work, but would be something Id need to check (and possibly pay to get amp modified?), but the kicker is that I wouldnt be getting the best out of them, so whats the point. :)
Great discussion btw, great info.
anthonyTD
21-12-2014, 19:32
There was another thread on AOS way back now, i cant remember, where it was explained about the extra heater current to someone else who was interested in the substitution,[also, this info has been posted on my own website for a good few years now; since I was amongst the first in the uk to mess about with them] and to be honest, its not a real issue with most well designed KT88 based valve amps,[KT88 1.85A Surge, warm 1.6A; KT120 1.95A surge, warm 1.75A approx] however, you were right in reminding people of the potential danger of just trying them without at least consulting the amplifiers designer, or failing that,reading up on as much info provided by those who have done so succesfully.
BUT....the KT120 draws between 100 and 300 mA more filament current than a KT88 so one should never directly substitute a KT120 for a kT88 before first checking with the manufacturer. Again, one must be mindful of generalising about these things when making recommendations. Valve amps do not conform to any sort of universal specification any more than SS amps do, so it is a dangerous thing to ever make assumptions about what is or isn't safe in terms of valve rolling. The danger here is that by doing so, an amp which isn't designed to take either valve will likely burn out part of the power supply windings due to overcurrent by dropping in a KT120 for a KT88. (There's also the question of circuit design and gain and distortion aside, some circuits may not gain the perceived sonic benefits even if the valves can safely be substituted but that's a subjective assessment for such amp owners to make for themselves as they may well prefer one to the other. I know of some who swear by NOS GEC KT88s and wouldn't use anything other than these). I do note your use of the qualification "...could..." though Marco
tubehunter
22-12-2014, 10:45
Marco, Nick undoubtably understands why KT120's make a diffrence as his explanation bears out, the output impedance of the amp is affected due to the GM, OR MA/V, diffrence, combined by the lower internal plate resistance of KT120's compared to KT88, This is the main reason you were able to lower the over-all feedback of the amplifier, gaining more presence, without sacrificing bass grip and control. The interesting one is the KT150, even though the Transconductance or MA/V is similar, or identical to KT120's the internal plate resistance/impedance is actualy higher than a KT88...
A...
Hi Anthony
Any chance you could explain 'gaining more presence'
Duncan
anthonyTD
22-12-2014, 11:00
Hi Duncan,
I think i did explain that earlier [sort of ] Anyway here goes' in the copper amps, with KT120's fitted less feedback is needed to maintain low end grip, therefore' by removing some of the feedback, the ampliers tend to project intermate, low level detail' more effortlessly at low to medium listening levels, without sacrificing over-all control, that's the best way i can describe it realy. :)
Hi Anthony
Any chance you could explain 'gaining more presence'
Duncan
Hi Dunc,
Until Anthony returns, I’ll have a go at that… It was the same in Ian’s Copper amp [btw, how is he - haven’t seen him for ages?] when the adjustable feedback control was fitted, and when the pot was used to reduce the level, quite a lot of feedback was taken off of the amp from what was originally set by the stock fixed resistors. Anthony can comment on exactly how much.
Fine-tuning the feedback level, by reducing it to the point where more “presence” was gained, i.e. overall openness, midrange expressiveness/vocal intelligibility - the sense that music was allowed to ‘breathe’ more/flow better, with a freedom of expression and incisiveness that was lacking previously, when the feedback level was higher, but without adversely affecting bass grip, so that along with the aforementioned midrange traits, bass lines were rendered as tight, tuneful and extended as possible.
With feedback, I find, it’s a matter of fine-tuning the overall balance, so that you obtain the best compromise between exerting sufficient ‘control’ on the music and allowing the amplifier, and subsequent music reproduced, to ‘breathe’. Too much feedback strangles the music, and too little makes for sloppy, ill-defined bass and a somewhat ’shouty' midrange, so in that respect, one has to find the ‘sweet spot’, which is why an adjustable control was fitted to the amp.
I guess that what Anthony’s saying is that fitting KT120s facilitated the achieving of that effect better than was possible with KT88s, due to the former’s specific electrical characteristics :)
Marco.
Ah, I see that Anthony’s gotten there before me! :)
Anthony, do you know what value the feedback resistors on the Copper amp were before the adjustable pot was fitted?
Marco.
tubehunter
22-12-2014, 12:24
Hi Marco/ Anthony,
Thanks for your replys, I'm just trying to get my head around negative feedback.
D T N Williamson the designer of the Williamson amp, which I believe the copper amps are based on, states in his description pubished in Wireless World 1950.
'Negative Feedback Network.
The design of this amplifier is such that no difficulty should be experienced in the application of negative feedback up to a maximum of some 30 db. Provided that the threshold of instability is not reached, the benefits of negative feedback increase as the amount of feedback is increased, at the sole expense of loss of gain, but there will be little if any audible improvement to be gained with this amplifier by increasing the amount of feedback beyond 20 db.'
Further more he goes on to Linearity
'Linearity.
http://i514.photobucket.com/albums/t350/halx00/art-107h_zpsad0df10e.jpg (http://s514.photobucket.com/user/halx00/media/art-107h_zpsad0df10e.jpg.html)
Oscillograms of input-output characteristic ; left-hand column, without feedback; right-hand column, with feedback. (1) At 300 Hz with slight overload (2) At 300 Hz, output voltage 15% below maximum. (3) and (4) Conditions as in (1) and (2) respectively, but at 30 Hz.
The linearity of the amplifier is well illustrated by the series of oscillograms. These show that, up to maximum output, the linearity is of a high order, and that the overload characteristic is of the desirable type shown in Fig. 1(b) in the previous article. The improvement due to the application of negative feedback, especially at low frequencies, is clearly demonstrated by the oscillograms.
http://i514.photobucket.com/albums/t350/halx00/art-107g_zps0861aa50.jpg (http://s514.photobucket.com/user/halx00/media/art-107g_zps0861aa50.jpg.html)
Fig. 6. Input-output characteristic and harmonic distortion curves, with and without feedback.
So it looks like Negative Feedback is required for low distortion results.
Radford Revival
22-12-2014, 13:07
So it looks like Negative Feedback is required for low distortion results.
Indeed. Luckily feedback is very good at achieving this!
anthonyTD
22-12-2014, 14:40
Hi Duncan/will,
Marco has described his findings better than i could, as for the data provided, i have to admit that i am a bit confused as to what its suposed to be explaning apart from Feedback improving low frequency distortion factors on the original williamson design ? bear in mind that the output transformers in those days where massive in comparison to the ones designed today' to acheive a decent amount of low frequency response,[due mainly to the poor steel available] which in turn' required mulitple interleavings to maintain a decent high frequency response, these days we have much better silicone steel, and therefore the transformers can be made much smaller in comparison, this has many positives as i am sure you will understand, ie; lower numbers of interleavings required, reduced phase shift etc, etc.
A...
Hi Anthony,
Can you remember how much feedback was applied to the Copper amp, in stock form, before the variable adjustment pots were fitted?
Marco.
anthonyTD
22-12-2014, 14:56
From memory, it was around 18db.
A...
Hi Anthony,
Can you remember how much feedback was applied to the Copper amp, in stock form, before the variable adjustment pots were fitted?
Marco.
So it looks like Negative Feedback is required for low distortion results.
You can build low distortion, relative, without feedback as well depending on the circuit topology and choose of o/p valve. IE: DHT And when talking about distortion its also worth thinking about even and odd order as they are quite different. Even order is not objectionable up to and above a few % but Odd order is quite horrid at the same levels..
anthonyTD
22-12-2014, 14:58
:)
You can build low distortion, relative, without feedback as well depending on the circuit topology and choose of o/p valve. IE: DHT And when talking about distortion its also worth thinking about even and odd order as they are quite different. Even order is not objectionable up to and above a few % but Odd order is quite horrid at the same levels..
From memory, it was around 18db.
Ah, ok, and can you remember how much adjustment was offered (in terms of reducing that amount of feedback) by using the varable mono adjustment pots fitted?
For example, the dials on the pots go from zero to nine, and I have mine sitting at just under five, with zero equalling the stock 18db level of feedback. Therefore, approximately how much feedback do you think has been removed from the amp, with the pots used at that setting? :)
Marco.
anthonyTD
22-12-2014, 15:25
The controls on your paticular copper amp only allow you to remove around 6db of feedback from each chanel [again from memory] i would also like to explain that the original copper amps were only loosely designed around the original williamson design topology, of which there were at least three versions that i know of, the first using true triodes, and two other versions using KT66, AND 807's connected as triodes, this is a very important aspect to remember, the copper amps use their output valves ultra-linear connected, and also have aditional feedback loops around each output valve to further linearize their operating conditions...
A...
Ah, ok, and can you remember how much adjustment was offered (in terms of reducing that amount of feedback) by using the varable mono adjustment pots fitted?
For example, the dials on the pots go from zero to nine, and I have mine sitting at just under five, with zero equalling the stock 18db level of feedback. Therefore, approximately how much feedback do you think has been removed from the amp, with the pots used at that setting? :)
Marco.
The controls on your paticular copper amp only allow you to remove around 6db of feedback from each chanel [again from memory]
Cool. So by having the variable control knobs set at just under 5 (the halfway position), I've removed approximately a further 3db of feedback (from the stock 18db), giving the amp currently a feedback level of 15db?
Would that make it a low, medium or high-feedback design? Sorry for all the questions - just trying to learn! :)
Marco.
Back to school for you Marco....18 - 3 is 15 mate....now write that 100 times please:eyebrows:
anthonyTD
22-12-2014, 16:52
Yes that would be a good approximation, as to what it would be classed as ie; low, medium, or high, i would say in genral terms it would be classed within the low to medium area, however, when compared to solid state designs, most valve amps can be considered low feedback...Due to the very diffrent nature of how each device works, and how the gain/versus frequency is controled/set etc.
A...
Cool. So by having the variable control knobs set at just under 5 (the halfway position), I've removed approximately a further 3db of feedack (from the stock 18db), giving the amp currently a feedback level of 16db?
Would that make it a low, medium or high-feedback design? Sorry for all the questions - just trying to learn! :)
Marco.
Ok, gotcha - cheers! :thumbsup:
Marco.
Back to school for you Marco....18 - 3 is 15 mate....now write that 100 times please:eyebrows:
Lol - now altered... Too much Drambuie!! :D
Marco.
The Black Adder
22-12-2014, 16:59
Mmmm... Drambuie :)
I'm switching between that and Armagnac. The Drambuie compliments Del's home-made Christmas cake, and the Armagnac, my dad's home-made all-butter shortbread...
Oh, decisions, decisions! :eyebrows:
Marco.
How do you all get the time for home made cake and home cooking in general, fabulous food and hours spent on here? I've had to put 'work' aside, including a home-hifi project, to help clean, shop and generally bustle around, and that's just so the three of us can enjoy Christmas and the rest of the hols together :)
I'm switching between that and Armagnac. The Drambuie compliments Del's home-made Christmas cake, and the Armagnac, my dad's home-made all-butter shortbread...
Oh, decisions, decisions! :eyebrows:
Marco.
Which Armagnac is that Marco? Janneau 'Napoleon'? You can only buy the latter these days in airport duty-free shops! :doh:
This stuff, old chap (from my fav vintners), and rather fine it is, too: http://www.tanners-wines.co.uk/spirits/brandy/armagnac/chateau-du-tariquet-xo-bas-armagnac-grassa-et-fils-70cl.html
:)
Marco.
How do you all get the time for home made cake and home cooking in general, fabulous food and hours spent on here? I've had to put 'work' aside, including a home-hifi project, to help clean, shop and generally bustle around, and that's just so the three of us can enjoy Christmas and the rest of the hols together :)
Lol - Del made the cake last weekend, and she cooks properly when she has time, which is at weekends and now that she's off on her Xmas hols for 16 days. I do lots of 'bits and pieces' in between when she's at work. Christmas shopping, apart from groceries, was done about 2 weeks ago.
We're all sorted now, apart from collecting the goose and turkey from the butchers, first thing on Chrtistmas eve (8am to avoid the rush), where we will then get the veg from the same indoor farmer's market, then it's scootle off back home to relax, ready for the big day! :cool:
Marco.
This stuff, old chap (from my fav vintners), and rather fine it is, too: http://www.tanners-wines.co.uk/spirits/brandy/armagnac/chateau-du-tariquet-xo-bas-armagnac-grassa-et-fils-70cl.html
:)
Marco.
Looks to be a good one Marco!
I can thoroughly recommend the Tesco 'Finest' VSOP Armagnac
http://img.tesco.com/Groceries/pi/792/5051140626792/IDShot_540x540.jpg
At £22 for a 70cl bottle, it makes an acceptable substitute if you have to ‘slum it’, owing to one’s 'man' having taken a little too long to track down the Janneau ‘Napoleon’:
https://sp.yimg.com/ib/th?id=HN.608007016912127585&pid=15.1&P=0
Cheers, Barry - I’ll bear that one in mind :)
Marco.
Well, despite all the KT88/120 schennanigans going on, and great chinwaggery it is too, I decided to go with a different valve after all.
300B. :)
:lol:
Well, despite all the KT88/120 schennanigans going on, and great chinwaggery it is too, I decided to go with a different valve after all.
300B. :)
Hallelujah praise the Lord
The best valve in existence. IMO
Valves rule, after the thread was hijacked for what to drink I am getting it back on topic now as after all AOS is supposed to be a high end hifi forum
is it not?
Smell my botty gas indeed! :lol:
http://www.aa-acoustics.com/resources/_wsb_375x356_psvane+300B+pair+crop+sf.jpg
have to say...
my hand was forced after my Oriental valve amp comitted Hari Kari....
though having heard a 300B B.E. (before Edingdales), was very impressed by the clarity, but not the whallop.
Eddingdales at 94dB and I dont have to worry.
Nick's sorting me out Dave. :)
have to say...
my hand was forced after my Oriental valve amp comitted Hari Kari….
Nightmare, mate, what happened?
...though having heard a 300B B.E. (before Edingdales), was very impressed by the clarity, but not the whallop.
Eddingdales at 94dB and I dont have to worry.
Nice one - 300Bs should match the Eddies really well :)
Marco.
Ali Tait
26-12-2014, 20:16
They do. Real synergy.
Nightmare, mate, what happened?
Nice one - 300Bs should match the Eddies really well :)
Marco.
As Saint and Greavsie used to say on a Saturday afternoon Marco...its a funny old game.
Was asking on the forum about the potential benefits of KT120's over my 88's...in the the other room...music in the background piping through from the listening room...and I heard an almighty rustling sound, followed by a loud hum then a bassy bang.
Went through, valve amp off with one valve glowing cherry red. :(
Had this exact problem with runaway bias before ON THE SAME VALVE SOCKET so thought, hmnn..
Replaced the fuse and the valve, on power up one channel loud hum then fuse blows again, but not without a flash inside the body of the amp and an accrid burning smell coming from the mains transformer :(
Dead as dogshit, as they say.
Inherant fault on the amp which showed its face for a second time taking out the mains trannie.
No one lost a leg, no fire or faulty RCD mechanisms all worked how they should.
Just bizaare how it went exactly when I was talking about changing valves.....
BUT...on to better things.
Really sorry to hear that, mate. Almost the same thing happened to me with a Chinese valve amp…
Trouble is, most of them (despite what manufacturer’s claim) can’t safely handle UK mains for prolonged periods of time, especially if the voltage where you live is on the higher side of the range. I know of some folk in the UK who’ve measured nearly 270V on their supply - and that ‘over-voltage’ (and even less than that), compared with the 220V the Chinese trannies would’ve been rated at, is more than enough to cause what happened to your amp! :rolleyes:
Did you get yours checked out in that respect by an independent engineer?
Marco.
Ali Tait
26-12-2014, 22:34
Secondary voltages were fine on UK mains Marco.
Interesting, Ali. Wonder what caused it, then? I think I’d always be inclined to use a step-down tranny with Chinese valve amps - just to be safe!
Marco.
Ali Tait
26-12-2014, 22:48
Don't know how old the amp was, or indeed exactly what went wrong with it, could be a few things.
Be interesting to perform a postmortem...
Ali Tait
27-12-2014, 10:18
Would seem to be connected to the runaway bias incident..
Interesting, Ali. Wonder what caused it, then? I think I’d always be inclined to use a step-down tranny with Chinese valve amps - just to be safe!
Marco.
Now you've gone round in a full circle mate, if you see where I'm coming from :lol: (I know, a step down transformer isn't the same as the 'other' discussion). Good idea though if the step-down is properly configured and made up.
Well whatever happened it was good news really as the 300B and edingdales are a match made in heaven as far as I am concerned.
Here are mine, not true edingdales but similar, Nick's 300B mono blocks sound stupendously good with them
http://www.mains-cables-r-us.co.uk/2911-thickbox_default/mcru-no139-loudspeakers.jpg
Nice speeks Dave, heard a set at Owsten, very nice.
Going down the power amp option rather than an integrated I think, offer more flexibility for future upgrades.
Marco, Nick undoubtably understands why KT120's make a diffrence as his explanation bears out, the output impedance of the amp is affected due to the GM, OR MA/V, diffrence, combined by the lower internal plate resistance of KT120's compared to KT88, This is the main reason you were able to lower the over-all feedback of the amplifier, gaining more presence, without sacrificing bass grip and control. The interesting one is the KT150, even though the Transconductance or MA/V is similar, or identical to KT120's the internal plate resistance/impedance is actualy higher than a KT88...
A...
You have me somewhat confused here Anthony, as far as I am aware , Tung Sol 's published valve characteristics data spec the KT120 and KT150 identically at 3000 Ohms :scratch:
anthonyTD
27-01-2015, 11:30
Hi Roger,
Datasheet below, a bit confusing, but you will see that they list the Tube impedance as 10 to 12k.
http://www.tubeampdoctor.com/images/File/data%20sheet%20KT150%20Tung-Sol.pdf
You have me somewhat confused here Anthony, as far as I am aware , Tung Sol 's published valve characteristics data spec the KT120 and KT150 identically at 3000 Ohms :scratch:
Hi Roger,
Datasheet below, a bit confusing, but you will see that they list the Tube impedance as 10 to 12k.
http://www.tubeampdoctor.com/images/File/data%20sheet%20KT150%20Tung-Sol.pdf
Anthony, Ahhh, I was working from the following pdf's impedance characteristics figures under load.
http://http://www.tungsol.com/tungsol/specs/kt150-tung-sol.pdf
http://http://www.tungsol.com/tungsol/specs/kt120-tung-sol-specs-curves.pdf
Would not any disparity in the respective Tube impedances only come into play were one looking at substituting KT120's with KT150's within the same amplifier/ circuit. I would expect that any difference in specification would become a moot point within an amplifier with a suitably modified bias circuit or perhaps more obviously we're the amplifier designed around the KT150 viz ARC's GS150 Galileo.
anthonyTD
27-01-2015, 15:32
Hi Roger,
I agree,
If the amplifier were designed around the tube then of course the diffrences in the quoted impedances would be taken into account, However, i think in this thread we were discussing swapping out KT88's for KT120's as a direct substitute.
A...
Anthony, Ahhh, I was working from the following pdf's impedance characteristics figures under load.
http://http://www.tungsol.com/tungsol/specs/kt150-tung-sol.pdf
http://http://www.tungsol.com/tungsol/specs/kt120-tung-sol-specs-curves.pdf
Would not any disparity in the respective Tube impedances only come into play were one looking at substituting KT120's with KT150's within the same amplifier/ circuit. I would expect that any difference in specification would become a moot point within an amplifier with a suitably modified bias circuit or perhaps more obviously we're the amplifier designed around the KT150 viz ARC's GS150 Galileo.
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